The present invention relates to a method of producing steel strip and the cast strip produced according to the method. In particular, the present invention relates to producing steel strip in a continuous strip caster. The term “strip” as used in the specification is to be understood to mean a product of 5 mm thickness or less.
The applicants have carried out extensive research and development work in the field of casting steel strip in a continuous strip caster in the form of a twin roll caster.
In general terms, casting steel strip continuously in a twin roll caster involves introducing molten steel between a pair of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the rolls to produce a solidified strip delivered downwardly from the nip, the term “nip” being used to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,934,359.
We have found that the concentration of residuals in the steel composition can effect the finished microstructure, and in turn affect yield strength and other mechanical properties of cast strip. In particular, higher concentrations of residuals make it possible to use lower cooling rates to transform the strip from austenite to ferrite in a temperature range between about 850° C. and 400° C. to produce microstructures in the cast strip that provide high yield strengths. It is understood that the transformation temperature range may be within the range between about 850° C. and 400° C. and not necessarily that entire temperature range. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
There is provided a method of producing steel strip which includes the steps of:                (a) continuously casting molten low carbon steel, silicon/manganese killed steel or aluminum killed steel, as defined below, into a strip including austenite grains, said molten steel comprising a concentration of residuals in the steel composition selected with regard to the microstructure of the strip that is required to provide desired mechanical properties, said residuals selected from the group consisting of copper, nickel, chromium, molybdenum and tin where the residuals are selected from the group in the amounts of more than 0.15 wt % copper, more than 0.08 wt % nickel, more than 0.08 wt % chromium, more than 0.03 wt % molybdenum and more than 0.015 wt % tin; and        (b) cooling the cast strip to transform the austenite grains in the strip to ferrite in a temperature range between about 850° C. and 400° C.        
The continuous caster may be a twin roll caster. The term “residuals” covers levels of elements of copper, tin, nickel, chromium, and molybdenum that are included in relatively small amounts equal to or less than about 2.0%, and are usually as a consequence of standard steel making as occurs in an electric arc furnace and/or ladle metallurgy furnace. The residuals are the result of purposeful additions through directly adding to the molten melt of a source or sources of the desired residuals in the electric arc furnace and/or ladle metallurgy furnace in the following amounts: more than 0.15% copper, more than 0.08% nickel, more than 0.08% chromium, more than 0.03% molybdenum and more than 0.015% tin. These percentages are all by weight percent, and are often abbreviated as “wt %”. The residuals in these amounts, which are greater than the weight percent of these elements found as impurities in typical steels, need not all be added to the molten steel to obtain the desired microstructure and resulting mechanical. Rather, the residual or residuals are selected from the group described to impart to the steel the desired microstructure and mechanical properties to the steel being made. In addition, in the case of low carbon steel, as defined below, where copper and tin are both used as residuals, the amount of copper plus tin must be ≧1.15%.
Alternatively, the residuals may be purposely added through the mix of scrap steel used to produce the molten melt in an electric arc furnace. Pig iron or another source of relatively high purity iron is typically added to the melted scrap in an electric arc furnace to dilute the amounts of copper, nickel, chromium, molybdenum, tin, and other impurities, found in the scrap when melted. The levels of these residuals in the melted scrap is the result of the mixture and amounts of the elements in the scrap. The purposeful addition of the residuals for the present invention can therefore be through the selection of scrap with higher levels of one or more of the residual elements, and then adjusting the amount of pig iron, as for example by purposefully adding to the melt lesser amounts of pig iron, or no pig iron, to achieve the desired levels of the selected residual elements to achieve the desired microstructure and mechanical properties in the molten steel. For this reason, cheaper sources of scrap and lesser amounts of relatively expensive pig iron can be used to produce steels with particular microstructures and mechanical properties if desired. Again, in the case of low carbon steel, as defined below, where copper and tin are both used as residuals, the amount of copper plus tin must be ≧1.15%.
These residuals may be up to about 2.0 wt % where harder cast steel strip is desired with yield strengths up to and in excess of 700 MPa. This weight percent is the total weight percent in the steel strip including the residuals from scrap steel and steel processing. In some embodiments, the total amount of the residuals may be 1.2 wt % or less. It should be noted that other residual elements, other than copper, nickel, chromium, molybdenum and tin, may be present as impurities in the steel, mostly from iron scrap, and can affect the microstructure and mechanical properties of the steel, but these other impurities are not purposefully controlled to achieve the desired microstructure and mechanical properties in the present invention.
In some embodiments, the cast strip produced in step (a) may have a thickness of no more than 2 mm.
In some embodiments, the cast strip produced in step (a) may include austenite grains that are columnar.
The steel may be low carbon steel, silicon/manganese killed steel or aluminum killed steel. The term “low carbon steel” is understood to be mean steel of the following composition, in wt %, that is not silicon/manganese killed steel or aluminum killed steel:
Carbon:0.02-0.08Manganese:1.0 or less;Silicon:0.5 or less;residuals:2.0 or less; andFe:balance.
The steel may be silicon/manganese killed, which has the following composition by weight:
Carbon:0.02-0.08%Manganese:0.30-0.80%Silicon:0.10-0.40%Sulfur:0.002-0.05%Aluminum:less than 0.01%residuals:2.0 or less; andFe:balance.
The steel may be aluminum killed, which has the following composition by weight:
Carbon:0.02-0.08%Manganese:0.40% maxSilicon:0.05% maxSulfur:0.002-0.05%Aluminum:0.05% maxresiduals:2.0 or less; andFe:balance.
The aluminum killed steel may be calcium treated. The method may further include the step of inline hot rolling.
Step (b) may include cooling the strip to transform the strip from the austenite to ferrite at a selected cooling rate of at least about 0.01° C./sec, and usually at least 0.1° C./sec, to produce a microstructure that provides required yield strength properties of the cast strip, the microstructure being selected from a group that includes microstructures that are:                (i) predominantly polygonal ferrite;        (ii) a mixture of polygonal ferrite and low temperature transformation products; and/or        (iii) predominantly low temperature transformation products.        
It is understood that most embodiments of the present invention will have microstructures of types (ii) and/or (iii).
The term “low temperature transformation products” includes Widmanstatten ferrite, acicular ferrite, bainite, and martensite.